1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, and more particularly to a highly durable electrophotographic photosensitive member excellent in mechanical strength, surface lubricating property, moisture resistance and imaging performance.
2. Related Background Art
The electrophotographic photosensitive member is required to have suitable sensitivity, electrical properties and optical properties matching the electrophotographic process to be employed. Also in a photosensitive member for repeated use, the surface layer, most distant from the substrate is subjected to electrical and mechanical actions for example in the steps of corona charging, toner development, image transfer to a paper sheet, cleaning process etc. and is required have resistance to such actions. More specifically there are required resistances to surface abrasion or damaging caused by friction, and surface deterioration caused by ozone generated by corona discharge in a high humidity condition.
On the other hand, toner deposition onto the surface layer is induced by the repetition of toner development and cleaning step, and the surface layer is required to have an improved cleaning property.
There have been proposed various methods for achieving these requirements, and one of such methods is the addition of lubricant. However the lubricant tends to migrate to the surface, so that the coated film becomes rich in the lubricant at the surface. Thus the photosensitive member shows good mechanical properties in the beginning, but soon loses said properties as the surface portion containing the lubricant is abraded off.
Another method consists of dispersing solid lubricant, particularly powdered fluorinated resin, in the surface layer of the photosensitive member, as disclosed in the Japanese Patent Laid-open Application No. 57-74748 and the U.S. Pat. Nos. 4,030,921 and 4,663,259. The dispersion of powdered fluorinated resin improves the resistances to surface damaging, surface cleaning and abrasion, and is effective also for surface deterioration under high humidity as it also improves the water-repellent property and releasing property of the surface of the photosensitive member. Also a protective layer containing powdered fluorinated resin dispersed therein further improves the durability, as the charge transporting material and the charge generating material, which are easily subjected to deterioration by ozone, are separated from the surface of the photosensitive member.
However, if the surface layer of the photosensitive member is formed by a coated film obtained by coating a liquid in which the powdered fluorinated resin is dispersed, said powdered resin is not exposed to the surface of thus formed coated film, and said surface is composed of the binder resin. Consequently the effect of the dispersed fluorinated resin cannot be seen in the beginning period of the use. As the result there are encountered various troubles such as the fusion of the surface of the photosensitive member and the cleaning blade, and the damage of said surface caused by the cleaning blade. For avoiding such troubles it is necessary to sprinkle toner or powdered lubricant on said surface or to intentionally scrape the surface off, and such operation inevitably raises the cost on the point of the required facility and labor.